With the recent progress of miniaturization and high performances of electronic devices, microminiaturization of conductor wiring has advanced so as to improve mounting density of electronic components in a multilayer wiring board, and the wiring forming technology thereof has been desired. There have been known, as the method for forming high-density fine wiring on an insulating layer, an additive method in which a conductor layer is formed only by electroless plating, a semi-additive method in which a thin copper layer is formed over the entire surface by electroless plating and then a conductor layer is formed by electroplating, followed by flash etching of the thin copper layer, and the like.
Usually, when through holes or blind vias required for in-layer connection of a printed wiring board are formed by laser processing, in order to remove smear generated during the formation, a desmear treatment is carried out, a neutralization step being carried out using a reducing agent after undergoing a wet type treatment with a swellant and an oxidizing agent such as an alkali permanganic acid solution in the desmear treatment. In the semi-additive method, the wet type treatment may be sometimes used for the purpose of forming a comparatively large physical anchor on a surface of an insulating layer to ensure the strength of adhesion with a conductor layer to be formed thereon, in addition to the purpose of the removal of smear.
Large roughness of the surface of the insulating layer makes it impossible to completely remove plating from the depth physical anchor in a flash etching treatment at a late process, and thus it is desired to make roughness of the surface of the insulating layer as small as possible. On the other hand, small roughness of the insulating layer tends to cause a decrease in strength of adhesion between the conductor layer and the insulating layer. Thus, there is a need to use, as a resin composition serving as a material of the surface of the insulating layer, a resin composition having high strength of adhesion with the conductor layer even if the surface of the insulating layer has small roughness.
With the progress of miniaturization and high density of a multilayer printed wiring board, intensive study has been made on thinning of a laminate used in the multilayer printed wiring board. With the thinning, problems such as deterioration of mounting reliability and increase in warpage of the multilayer printed wiring board may arise, and thus it is required for the resin composition serving as the material of the insulating layer to have low thermal expansion and high glass transition temperature.
In order to solve such problems, Patent Literatures 1 to 4 mention technologies using, as the component of the resin composition serving as the material of the insulating layer, a rubber component and/or a filler which undergo(es) decomposition, falling-off or dissolution in an alkali permanganic acid solution which is an oxidizing agent during a desmear treatment. These literatures disclose that decomposition, falling-off or dissolution of the rubber and/or the filler cause fine unevenness on the surface of the insulating layer during the desmear treatment, leading to high force of adhesion between the conductor layer and the insulating layer, but make no mention of the concept of high glass transition temperature.
Patent Literature 5 discloses, as the resin component of the insulating layer, a resin composition containing a cyanate resin and/or a prepolymer thereof, an epoxy resin, a phenoxy resin, an imidazole compound and a filler. The resin composition achieved high plated copper peel strength of the conductor layer formed by plating, but did not achieve satisfactory low roughness and low thermal expansion.